1. Field of the Invention
The present invention relates generally to optical communications through layers of printed circuit board (PCBs).
2. Discussion of the Related Art
Multi-layer PCBs are required due to the large number of electrical signals and the necessarily large number of signal paths. Limitations exist with this prior art technology with regard to how closely spaced electrical signal paths can be placed. Spacing limitations are a result of increasing signal speeds, potential metal migration problems (growth and shorting as a result of voltage differentials), and manufacturing specifications. Also, the length of a circuit from one device to another becomes a concern as circuit input/output speeds increase. Thus, device connection lengths become important concerns with limitations that are, at times, very difficult to overcome. The prior art also utilizes plating and etching processes which can be prone to manufacturing defects such as over-etching a circuit (potential open circuit or future open circuit) or under-etching, which has the potential to have a short circuit from one circuit to another (or future short). Systems which use electrical signals to transmit information from one chip to another on the same printed circuit board, or to another printed circuit board (PCB) with an electrical backplane, typically use multi-layer printed circuit boards in order to route the numerous signals on the PCB.
Present day multi-layer PCBs are manufactured with both surface and multiple internal layers of circuit carriers, commonly referred to as inner-planes. Via holes, usually copper, are used to carry signals from one location to another through the surface and inner-plane circuit connections. Via holes are also used to connect surface mount devices to inner-planes. Circuit pads on the surface layers are used for surface mounted devices (SMDs) and larger via holes are used to connect devices with through-pins from the device to another signal layer in the same PCB. Internal via connections are also used to internally connect a signal from one inner-plane to another inner-plane.
Making via holes with an electrically conductive lining is not suitable when optical signals are introduced to the surface of a multi-layer PCB that needs to be coupled through the PCB to another location, for example, to an internal optical waveguide.